This invention relates to a circuit for detecting the presence of a carrier signal on a transmission line and, more particularly, relates to a carrier detection circuit suitable for fabrication as a single integrated circuit, or a portion of a larger integrated circuit, requiring no off-chip components.
Modern developments in telecommunications have led to distributed processing, the widespread use of remote terminals and a plethora of stand alone data transmission/receiving stations which communicate with other data transmission/receiving stations. In many cases the individual stations are not manned. Even if they are manned, the information which is received is in an electronic format which is not directly intelligible to the human operator. It is necessary, therefore, for the equipment at each individual station to be able to sense when a carrier signal is being received. This permits the equipment to be readied to receive data. During the times when no carrier signal is being received the individual station may be used for other functions or the human operator may carry out tasks at other locations.
Prior art techniques of ascertaining the presence of a carrier signal in a transmission include measurement of the power level of a carrier signal. These approaches have invariably required the assembly of discrete components and have employed complex signal processing techniques. See, e.g., M. Ahmed, et al, "Carrier Detection Circuit", U.S. Pat. No. 3,746,993; and A. P. Clark, et al, "Detection Processes for a 9600 Bit/Second Modem", The Radio & Electronic Engineer, v. 51, p. 544 (1981), which are hereby incorporated by reference. Carrier detection circuits necessarily require sensitivity to very small signals and have very long time constants. Consequently, previous implementations have either required the use of well controlled off-chip components such as discrete capacitors and resistors and an attendant increase in the number of package leads, or have required the use of digital to analog converters with digital processing circuitry, which requires a large integrated circuit chip area. In order to provide a carrier detection circuit useable with portable terminals, with remote stations, or with modems, it is highly desirable to have a circuit which is capable of being manufactured on a single integrated circuit chip with small die area, reduced number of package leads, and no external components required in order to establish the required time constants.